CommunityMember Class Hierarchy

Let’s develop a simple inheritance hierarchy with five levels (represented by the UML class diagram in Fig. 11.2). A university community has thousands of CommunityMembers.

Fig. 11.2. Inheritance hierarchy for university CommunityMembers.

These CommunityMembers consist of Employees, Students and alumni (each of class Alumnus). Employees are either Faculty or Staff. Faculty are either Administrators or Teachers. Some Administrators, however, are also Teachers. We’ve used multiple inheritance to form class AdministratorTeacher. With single inheritance, a class is derived from one base class. With multiple inheritance, a derived class inherits simultaneously from two or more (possibly unrelated) base classes. We discuss multiple inheritance in Chapter 21, Other Topics, but multiple inheritance is generally discouraged.

Each arrow in the hierarchy (Fig. 11.2) represents an is-a relationship. For example, as we follow the arrows in this class hierarchy, we can state “an Employee is a CommunityMember” and “a Teacher is a Faculty member.” CommunityMember is the direct base class of Employee, Student and Alumnus. In addition, CommunityMember is an indirect base class of all the other classes in the diagram. An indirect base class is inherited from two or more levels up the class hierarchy.

Starting from the bottom of the diagram, you can follow the arrows upwards and apply the is-a relationship to the topmost base class. For example, an AdministratorTeacher is an Administrator, is a Faculty member, is an Employee and is a CommunityMember.